Page 6: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Formaldehyde

Classification and Assessment

Formaldehyde is a normal product of metabolism and is essential in the biosynthesis of certain amino acids. Rats exposed to formaldehyde by inhalation exhibited an increased incidence of carcinomas of the nasal cavity at doses that caused irritation of the nasal epithelium. Concentrations that are carcinogenic are also cytotoxic and increase cell proliferation in the nose.

However, the weight of evidence indicates that formaldehyde is not carcinogenic by the oral route. Ingestion of formaldehyde in drinking water for two years caused only stomach irritation in rats in several studies, particularly at higher doses.

The tolerable daily intake (TDI) of formaldehyde can be derived by dividing a NOAEL in an animal studyFootnote 48 by an appropriate uncertainty factor. For formaldehyde, the TDI is derived as follows:

Figure 1. Tolerable daily intake (TDI) equation

Figure 1


  • 15 mg/kg bw per day is the NOAEL for various effects in male rats, including pathological changes in the stomach and an increased incidence of renal papillary necrosis, in a study in which rats were exposed to formaldehyde in drinking water for two yearsFootnote 48
  • 100 is the uncertainty factor (×10 for intraspecies variation; ×10 for interspecies variation).

Since formaldehyde is metabolized quickly in the body and induces tumours only in animals exposed by inhalation at high cytotoxic doses, and as there is no clear evidence of carcinogenicity by the oral route, an extra uncertainty factor was not used.

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